The present invention relates generally to voltage-controlled oscillators (VCOs), and more particularly a voltage-controlled oscillator for use in a communications device, such as a modulator or receiver.
In all VCO's, phase noise degrades the performance of the VCO. Newer designs of VCOs have improved the phase noise performance, consequently previously unimportant contributions to the phase noise are now more significant. It is therefore desirable to further reduce the phase noise.
One technique for reducing the phase noise is to provide an external (i.e., off-chip) filter on the supply voltage line. For example, a resistor-capacitor combination (R/C) can be coupled externally to the power supply line. However, this technique reduces the supply voltage or the supply current for a given supply voltage to the VCO, thereby degrading performance of the VCO.
Alternatively, it has been attempted to add an external inductor-capacitor (L/C) combination to avoid the voltage drop inherent in the above device. Unfortunately, this technique results in little improvement below the cutoff frequency (f.sub.c) of the device, resulting in unrealistic component values.
Another technique for reducing phase noise is disclosed in U.S. Pat. No. 4,621,241, in which elaborate arrangements are made to reduce the phase noise. To improve the phase noise of the circuit, a series capacitor is included in a feedback loop, which includes impedance not present in the Colpitts or Clapp oscillators. Additional impedance (in the form of a resistor) is connected between the feedback output of the amplifier and ground. In this patent, low-pass filtering of noise is accomplished using a resistor to ground and a blocking capacitor in the feedback path. However, the values of the blocking capacitor and resistor used for the filter must be selected for a variety of purposes, which requires the circuit designer to choose less than the optimum values for these elements depending on the requirements of the circuit.
The present invention is therefore directed to the problem of developing a method and apparatus for improving the phase noise performance of a VCO without reducing the supply voltage or the supply current to the VCO.